The present invention generally relates to a method of measuring temperature and an apparatus employed for accomplishing this method, and more particularly, to a method and an apparatus for measuring temperature, for example, the temperature within a furnace, the temperature of a molten material, such as a molten iron or the like, by making use of a stage change resulting from a temperature change of a fluid.
Conventionally, a thermocouple, a resistance thermometer or the like has been widely employed for measurement of a high temperature within a boiler, a furnace or the like. However, since these kinds of thermometers are, in principle, restricted in material of a temperature-sensitive portion exposed to the high temperature, it has been difficult to take any countermeasure against oxidization or other causes shortening the life of the thermometer, and accordingly, such thermometers are generally improper to be used for a long time.
Accordingly, there has been developed a temperature measuring apparatus of fluidic resistance type or a fluidic pyrometer which offers an advantage such that a material of a probe, i.e. a temperature sensor forming a temperature-sensitive portion, can be freely selected in view of its life without any restriction by a measuring principle. As for the principle of the fluidic resistance type temperature measuring apparatus, the temperature is detected through a change of pressure drop of a gas at the time when it passes through a throttle portion such as a capillary tube, by making use of a temperature dependence of a viscosity coefficient of the gas. FIG. 1 illustrates a fundamental construction of the temperature measuring apparatus of the above described type in which a working fluid, such as Ar gas or the like, is initially supplied from a source S' of the working fluid at a constant pressure through a pressure control device 40. The pressure drop .DELTA.P across the capillary tube 42 within the probe 41 which arises in correspondance with the temperature of an atmosphere to be measured is detected as a pressure difference .DELTA.Pc between the pressure on the secondary side of a trim valve 43 and the pressure on the secondary side of the capillary tube 42. Thereafter, upon amplification of the pressure difference .DELTA.Pc by a fluidic element 44, the pressure difference is detected as an electric signal by a pressure transducer 45.
Such a system is substantially similar in construction to a kind of electric circuit called a Wheatstone bridge, and a slight fluctuation of the pressure drop at a sensitivity set valve 46, an amplifier supply valve 47, or the trim valve 43 exerts a large influence upon the pressure signal from the fluidic element 44. Accordingly, a state change of the working fluid caused by an environmental temperature causes the fluctuation with respect to the pressure drop at each of the aforementioned valves 46, 47 and 43. Since this fact is, in appearance, regarded as a fluctuation of the pressure drop .DELTA.P at the capillary tube 42 within the probe 41, i.e. a change in temperature detected by tne probe 41, the temperature measuring apparatus of this kind has a disadvantage because it can be subjected to the influence of the environmental temperature.
In addition, in the aforementioned fluidic resistance type temperature measuring apparatus, when the probe 41 as the temperature-sensitive sensor is damaged, for example, it is cracked or a hole is accidentally made in it causing the working fluid to spill out or the atmosphere to enter the probe 41, the signal outputted from the pressure transducer 45 will continuously output a signal that does not correctly correspond to the temperature being measured.
The damage of the probe in the fluidic resistance type temperature measuring apparatus is substantially equivalent to burnout with respect to the thermocouple. However, although no signal is outputted in the case of burnout, the wrong signal is continuously outputted in the fluidic resistance type temperature measuring apparatus. It is, therefore, difficult to detect the damage of the probe 41, and in the case where the temperature is controlled through its measurement, for example, by the fluidic resistance type temperature measuring apparatus, the temperature will be controlled undesirably to a value different from the predetermined one. This is another shortcoming of the fluidic resistance type temperature measuring apparatus.
On the other hand, in the case where the temperature within the furnace is controlled, the temperature measurement is generally executed simultaneously at a plurality of locations within the furnace. Accordingly, when the aforementioned fluidic resistance type temperature sensor is employed in a multi-temperature measuring apparatus, it is considered, as shown in FIG. 2, that the plural sets of the fluidic resistance type temperature sensors are connectively juxtaposed with each other, with the source S of supply of the working fluid and the pressure control device 40 being commonly used between them. Such construction, however, undesirably produces some new problems different from the aforementioned ones.
A first problem is that since the environmental temperatures are different for each of the locations or points where the amplifier supply valves 47, sensitivity set valves 46 and trim valves 43 generating the reference pressure drop are provided, all of these valves being located at the upstream side of each probe, the points to be measured in temperature undergo influences by the environmental temperatures which differ from each other. In other words, there occur measurement errors because of the temperature difference from one location to another measurement location making it impossible to correct the measurement errors.
A second problem is that it is impossible to supply the working fluid to each probe at a constant pressure. More specifically, the source S of the working fluid and the pressure control device 40 are commonly used, and therefore, since the pipings for supplying the working fluid to each probe 41 is inevitably long, a pressure fluctuation is produced in the working fluid while in the pipings because of the influence of the environmental temperature. As a result, this phenomenon causes a large measurement error in temperature.
As shown in FIG. 3, it is possible to supply the working fluid to each probe at the constant pressure, by restricting the pressure fluctuation occurring in the pipings being restricted. This restriction is accomplished by using additional correcting pressure control devices 40a for provided immediately before each temperature sensor unit. However, since the pressure fluctuation on the primary side of each pressure control device 40a is large, the pressure cannot be fully controlled. Accordingly, not only the measurement error in temperature becomes undesirably large, but also a plurality of the pressure control devices for correction use are inevitably needed. The number of these extra pressure control devices needed corresponds to the number of points used to measure temperature. These additional devices cause the temperature measuring apparatus to be manufactured at an undesirably increased cost, thereby causing the third problem.
A fourth problem is that since the multi-temperature measuring apparatus as shown in FIG. 2 or 3 has a plurality of the temperature sensor units that are juxtaposely connected with each other at the downstream side of the source S of the working fluid, it is necessary to supply the working fluid at a constant pressure to each unit, thereby causing the consumption of the working fluid to undesirably increase proportionately to the increased number of the points used for measuring.